Phylum Platyhelminthes (The Flatworms)

Phylum Platyhelminthes:

• Platyhelminthes is a phylum of dorsoventrally flattened acoelomate but triploblastic animals having bilateral symmetry and blind sac body plan.
• Due to their flattened nature, Platyhelminthes are also called flatworms.
• The term Platyhelminthes was coined by Gegenbaur (1859).

Characteristics of Phylum Platyhelminthes:

• Flatworms are free-living commensal or parasitic organisms. The free-living worms may be terrestrial or freshwater but a majority of endoparasitic.
• The body is dorsoventrally flattened, leaf-like or ribbon-like.
• The parasitic worms bear suckers and hooks to get attached themselves with hosts.
• Platyhelminthes are triploblastic animals with three germ layers- ectoderm, endoderm and mesoderm.
• Spaces between the body organs are filled with a mesodermal connective tissue, called parenchyma.
• Flatworms exhibit a definite organ system level of organisation. Flatworms are the first and the simplest animals that have organs and organ systems. The systems are simple.
• Excretion and osmoregulation occur by peculiar cells called flame cells or protonephridia or solenocytes. These are ammonotelic.
• The alimentary canal is branched or unbranched with an opening called a mouth.
• The nervous system includes a pair of an anterior ganglion (brain ganglion) and a ventral nerve cord.
• A skeletal system is absent. Flatworms are, therefore, soft. In certain cases scleroprotein cuticle is present.
• Flatworms have definite anterior and posterior ends. The concentration of sense organs at the anterior end marks the beginning of cephalization.
• Most of the flatworms are hermaphrodites but can reproduce asexually and by sexual methods. In sexually reproducing organisms, cross-fertilization occurs in most cases.
• Fertilization is internal and development is indirect through many larval stages.
• The power of regeneration is well marked in some flat worms.

Examples- Taenia Solium (Tapeworm), Fasciola (Liver Fluke), Echinococcus (Dog Tapeworm) and Planaria.

Advancement Over Coelenterates:

• Formation of organs.
• Cephalisation.
• Three germ layers.
• Polarized ladder-like a nervous system.
• Gonads with gonoducts.
• Copulatory organ.
• Flame cells as excretory structures.
• The occurrence of muscles in the body wall and gut wall.
• Bilateral symmetry.

• Characteristics of Phylum Porifera
• Chlorophyta- Green Algae
• Rhodophyta- Red Algae
• Phaeophyta- Brown Algae
• Bryophytes- Characteristics And Economic Importance
• Transpiration In Plants
• Hatch and Slack Cycle (C4 Pathway)
• Tamil Board: Living World